<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
</head>
<body>
    <input type="text" placeholder="半径" id="a">
    <input type="text" placeholder="周长" id="b">
    <input type="text" placeholder="面积" id="c">
    
    <script>
        
        var p=Math.PI;
        console.log(p);
        
        var r=prompt("请输入半径");
        r=Number(r);
        
        document.querySelector("#a").value=r;
        document.querySelector("#b").value=r*2*p;
        document.querySelector("#c").value=r*r*p;
        
        
        
    </script>
    
</body>
</html>